Nitrogen oxides formed in combustion lead to irritation and damage to the respiratory organs (especially in the case of nitrogen dioxide), and formation of acid rain due to formation of nitric acid. In the removal of nitrogen oxides from flue gas (also known as DeNOx), nitrogen oxides such as nitrogen monoxide (NO) and nitrogen oxides (NOx) are, for example, removed from the offgas of coal-fired or gas turbine power stations.
As measures for removing nitrogen oxides from the offgases, reductive processes such as selective catalytic processes (selective catalytic reduction, SCR) are known in the prior art. The term SCR refers to the technique of selective catalytic reduction of nitrogen oxides in offgases from firing plants, domestic waste incineration plants, gas turbines, industrial plants and engines.
Many such catalysts contain TiO2, with the TiO2 acting as catalyst itself or acting as cocatalyst in combination with transition metal oxides or noble metals. The chemical reaction over the SCR catalyst is selective, i.e. the nitrogen oxides (NO, NO2) are preferentially reduced while undesirable secondary reactions (for example the oxidation of sulfur dioxide to sulfur trioxide) are largely suppressed.
There are two types of catalysts for the SCR reaction. One type consists essentially of titanium dioxide, vanadium pentoxide and tungsten oxide. The other type is based on a zeolite structure. Further metal components are also added to the two systems in the prior art.
In the case of TiO2—WO3—V2O5 catalysts, the V2O5 serves primarily as catalytically active species on WO3-coated TiO2 (in the anatase modification). The WO3 coating on the TiO2 is intended to function as barrier layer to prevent diffusion of vanadium into the TiO2 and the associated decrease in activity and formation of rutile.
WO3-doped TiO2 is proposed for catalytic applications, including as DeNOx catalyst, according to the prior art as per U.S. Pat. No. 4,085,193. The process known therefrom is based on the addition of tungsten components to a titanium component such as metatitanic acid, a titanium oxyhydrate or titanium dioxide suspension) and subsequent calcination to set the surface area to about 100 m2/g.
However, an additional, complicated milling step is often required before the further processing of the catalyst raw material. This is due to the fact that the tungsten-containing titanium dioxide material leaving the calcination furnace or after the heat treatment in the range from 150° C. to 800° C. is in the form of agglomerates in which the individual particles are joined to one another by sintering bridges or similar connections. In the case of relatively high-quality catalysts, in particular catalyst honeycombs having a very low web thickness, or application of the tungsten-containing titanium dioxide in the case of a “washcoat” to honeycomb bodies, milling is indispensable.
In industry, this milling process is usually carried out in a pendulum mill, e.g. a Raymond mill.
DE 102008033093 describes a process for producing a catalyst material comprising an optionally tungsten-containing titanium dioxide material, in which a titanium dioxide-containing catalyst material is produced as intermediate by milling in a roll mill, in particular a Gutbett roll mill, and the flakes leaving the roll mill as intermediate are not subjected after milling, in particular immediately afterward, to any deagglomeration and/or dispersing treatment.
The porosity of the catalyst is also of critical importance to the catalytic activity of a titanium dioxide-containing catalyst for the selective removal of nitrogen oxides from exhaust gases and offgases in the presence of ammonia. Thus, EP 516262 describes a shaped porous support composed of titanium dioxide particles alone or of a mixture of titanium dioxide particles with particles of another, porous, inorganic oxide, where the shaped support has a total porosity of 0.8 cm3/cm3 which is made up of a microporosity encompassing pores having a pore diameter of 60 nm or less of from 0.05 to 0.5 cm3/cm3 and a macroporosity encompassing pores having diameters greater than 60 nm of from 0.05 to 0.5 cm3/cm3. This catalyst support is preferably produced by mixing of materials which can be burnt out with titanium dioxide particles and shaping this mixture.
The object of the invention is thus to provide a catalyst material which displays improved properties compared to the materials known in the prior art.